US Air Force's X-51A Waverider scramjet sets hypersonic flight record

After flying at five times the speed of sound for more than three minutes Wednesday, the US Air Force's experimental X-51A Waverider scramjet has set a new record for the longest hypersonic combustion ramjet-powered flight.

ByTariq Malik, SPACE.com Managing EditorMay 27, 2010

In this image provided by the US Air Force, an X-51A Waverider rides under the wing of a B-52 Stratofortress, on Dec. 9, 2008. A similar X-51A successfully launched from a B-52 Stratofortress, Wednesday, and accelerated to Mach 6. It was the longest supersonic combustion ramjet-powered hypersonic flight to date.

An experimental aircraft has set a new record for the longest hypersonic flight after streaking across the sky Wednesday for more than three minutes while flying at Mach 5 – five times the speed of sound — the United States Air Force has announced.

Wednesday's test flight was aimed at evaluating the X-51A's scramjet engine, thermal protection, stability and control, and other systems.

With a profile that gives it a shark-like look, the X-51A scramjet cruiser is 14 feet (4.2 meters) long and is virtually wingless. It is designed to ride the shockwave it creates during flight, leading to its nickname "Waverider," the Air Force has said.

Pratt & Whitney Rocketdyne built the SJY61 scramjet engine at the heart of the X-51A cruiser.

Four seconds into the flight, the X-51A's solid rocket booster – actually an adapted Army Tactical Missile booster – accelerated the experimental aircraft to Mach 4.8 before being jettisoned to let the scramjet engine take over.

After the flight, the vehicle was expected to splash into the ocean. There were no plans to recover the craft, according to the Air Force.

"Now we will go back and really scrutinize our data. No test is perfect, and I'm sure we will find anomalies that we will need to address before the next flight," Brink said. "But anyone will tell you that we learn just as much, if not more, when we encounter a glitch."

The test was actually the third flight of the X-51 vehicle; it was the first time it flew independently. It had remained attached to its mother ship on both earlier flights.

Three more X-51 hypersonic tests flights are scheduled for later this year. The Air Force has built four X-51A cruisers in all, with one of them now successfully flown.

Hypersonic flight, typically defined as beginning at Mach 5, is more challenging than supersonic flight at lower speeds because of the higher temperatures and pressures involved with the faster flight speed. The speed of sound, Mach 1, is about 760 mph (1,223kph) at sea level.

But scramjets, air-breathing jet engines driven by supersonic combustion, like the one on X-51A have their own challenges too. Air Force project officials compared it to "lighting a match in a hurricane and keeping it burning."

The X-51A is not the U.S. military's only project undergoing tests this year.

The Air Force Research Laboratory and scientists in Australia successfully launched an unmanned suborbital rocket in March during a joint hypersonic flight test that hit Mach 5.5.

In late April, an unmanned hypersonic glider called HTV-2 failed during its test flight after blasting off on a rocket launched from Vandenberg Air Force Base in California. That test, too, was aimed at learning more about the intricacies of hypersonic flight to support the development of future aircraft technology.

The Air Force also launched the secretive X-37B space plane into orbit in late April from the Cape Canaveral Air Force Station in Florida. The exact mission of that reusable space plane, which is smaller than NASA's space shuttles but entirely robotic, is closely guarded. But amateur skywatchers have managed to spot the mysterious X-37B in the night sky using telescopes equipped with video cameras.